Our bodies are crawling with trillions of bacteria, fungi and viruses, and now it seems no corner of our anatomy is immune to microbial colonies. Researchers have discovered that the placenta, once thought to be sterile, actually plays host to over 300 types of bacteria.

More surprisingly, the types of microbial life living in the placenta, an organ that supplies a developing fetus with nutrients and filters waste, bear a striking resemblance to the bacterial biome of the mouth. The finding has fueled renewed interest in the link between periodontal disease in pregnant women and premature births.

Probing Placentas

Researchers from Baylor College of Medicine and Texas Children’s Hospital explored the placenta’s microorganisms as part of a massive, nationwide undertaking to catalog and study all the microbes living in and on the human body.

They studied 320 placentas collected after birth and sequenced bacterial DNA extracted from the organs. Their analysis revealed small populations of the harmless gut-dwelling E. coli, as well five other phyla of benign bacteria living in the not-so-sterile organ.

Researchers then compared the placentas’ bacterial DNA inventory to that of women’s skin, airways, vaginas, guts and mouths to deduce the microbes’ origin. They found that the placenta’s microbial mix most closely matched the cast of characters in the mouth.

Revisiting an Old Debate

The striking similarity between bacteria in the mouth and the placenta led researchers to hypothesize that the microbes migrated, via the blood stream, from mothers’ mouths to the placenta and ultimately to the fetus. Past studies have linked a labor-inducing chemical produced by oral bacteria, called prostaglandin, to premature births and low birth weight.

However, researchers said it’s too early to determine whether gum disease affects pregnancy outcomes — only one woman in their study had it. The most pressing questions will be to deduce the role bacterial colonies play in the placenta and how they got there.

At the very least, the findings prove that humans are great hosts for myriad microscopic communities.